Fuel level gauges are used to determine the filling height in a fuel tank and from that, in turn, the filling volume of the tank. Such devices are known in a variety of configurations. Fuel level gauges, for instance, for vehicles, are relatively simple in design and have fuel level sensors, for example, in the form of simple float constructions or the like. However, fuel level gauges for applications in aviation are quite complex in design.
This is due to the fact that, among other things, it has to be possible to refuel aircraft—as a function of the type of aircraft and of the scope of its mission—under all kinds of different conditions, a process in which high requirements are made of the precision of the fuel level gauges. Hence, it has to be possible to carry out refueling on the ground, air-to-air refueling (so-called in-flight refueling by means of a tanker aircraft) or, in the case of a rotary-wing aircraft, even so-called hover-in-flight refueling. Hover-in-flight refueling can be done, for example, from a ship above which the rotary-wing aircraft is hovering. Refueling on the ground makes use of so-called gravity refueling, in which the fuel is pumped into the tank through the effect of gravity or at a very low pressure, and also makes use of so-called high-pressure refueling (referred to below in short as pressure refueling), in which the fuel is filled into the tank by means of a pressure refueling device at a pressure of approximately 2 to 10 bar. In-flight refueling and hover-in-flight refueling fundamentally call for the latter modality of refueling, that is to say, high-pressure refueling. The (pressure) refueling of an aircraft is a safety-critical procedure. For this reason, the refueling operation has to be continuously checked. Here, in particular, the current filling level, the filling volume and the filling weight of the tank have to be constantly monitored. This serves for controlling or regulating and monitoring the refueling operation itself as well as for determining and checking important flight-mechanical parameters such as, for example, the fuel weight resulting from the filled fuel volume and, in turn, the maximum permissible take-off weight and flying weight as well as the maximum attainable range and flying time of the aircraft in question. To this end, the (pressure) refueling device and the fuel level gauge can normally be coupled to a fuel management system that, especially on the basis of the measured data acquired by the fuel level gauge, performs the necessary calculations and, if applicable, forwards this data to an on-board computer of the aircraft or to additional control devices.
A prior-art fuel level gauge of an aircraft has a measuring pipe configured as a riser pipe that has a fuel inlet opening at one end and that defines a measuring distance, and it also has a fuel level sensor associated with the measuring pipe in order to measure the fuel filling height in the measuring pipe. Normally speaking, a capacitive sensor is used as the fuel level sensor. This sensor functions according to the following principle: depending on the filling height of the fuel in the measuring pipe and the related degree of wetting of the measuring pipe surface with the fuel, a change occurs in the capacitance that—as a measurable quantity—represents a measure of the fuel filling height in the tank.